Echelle spectrometer

ABSTRACT

A spectrometer having an entrance aperture, at least one collimating mirror, a prism and an echelle grating which is so mounted to provide rotation in two directions thereby providing adjustment of the vertical and horizontal components of the dispersed energy in the exit focal plane.

United States Patent Elliott [54] ECHELLE SPECTROMETER [72] Inventor:William G. Elliott, Lincoln, Mass.

[73] Assignee: SpectraMetrics, Incorporated, Burlington,

Mass.

[22] Filed: Jan. 14, 1971 [2 1] Appl. No.: 106,561

7 Related US. Application Data [63] Continuation of Ser. No. 710,881,Mar. 6, 1968, aban- Mack: The Review of Scientific Instruments, vol. 15,No. 2,

ICOLLIMATING MIRROR 22 COLLIMATING MIRROR EXIT FOCAL PLANE February1944, pages 28- 36 Tolansky: High Resolution Spectroscopy, PitmanPublishing Corporation, 1947, pages 23 l- 237 Harrison: Journal of theOptical Society of America, vol. 39, No.7, July 1949, pages 522,- 528.

Hurrison et al.: Journal 01 the Optical Society of America. vol. 42. No.l(l,-()ct. 1952 pages 706-712 Bausch & Lomb Catalog D- 260, EchelleSpectrographs, pages 1- 24 v v Tarasov: Optics and Spectroscopy, vol. 11, No. 5- 6, Nov.-Dec. 1961, pages 368 and 369.

Primary Examiner-Ronald L. Wibert Assistant E.raminer-F. L. Evans vAttorney-Richard P. Crowley, Phillip G. Kiely and Richard L. Stevens [57] ABSTRACT A spectrometer having an entrance aperture, at least onecollimating mirror, a prism andan echelle grating which is so mounted toprovide rotation in two directions thereby providing adjustment of thevertical and horizontal components of the dispersed energy in the exitfocal plane.

23 Claims, 4 Drawing Figures SHAFT l4 SHAFT l6 PRISM ll GRATING l2PERTURE SLIT 21 PAIEN TEIlIPIIzs M2 3,658,423

SHEET 10F 2 SHAFT UPRIGHT SHAFT I4 /SHAFT MOUNT PRISM I/ /PI ATEMouNTING r fi PLATE|8 I INCIDENT I I ENERGY I I PRIsM II ,GRATIN f-PLATE DISPERSED *GRATING I2 ENERGY --BASE I3 F I G. I

GRATING I2 SHAF UPRIGHT I MouNTING PLATE l9 ,/SHAFT I6 PRIsM J II IINCIDENT ENERGY I 9 PRIsM MOUNTING SHAFT I4 PLATE DISPERSED ENERGY /BASEl3 F 2 INvENToR WILLIAM G. ELLIOTT BYM M ATTORNEYS r l ECHELLESPECTROMETER CROSS-REFERENCE TO RELATED APPLICATIONS This application isa streamlined continuation application of US. Pat. application Ser. No.710,881, filed Mar. 6, 1968, now abandoned.

BACKGROUND OF THE INVENTION Spectrometers are devices employed formeasuring the spectral energy distribution impinging on its entranceaperture. In general, there are two types; the dispersive type whichcauses energy to be concentrated in space as a unique function ofwavelength, and the interferometer type which produces interferencepatterns distributed in space as a function of wavelength. Thedispersive type must be used in any situation where it is necessary toisolate a particular wavelength interval, i.e., in experiments whichdepend upon the energy per photon of impinging energy. Two methods areconventionally used to provide the dispersion; a prism or a grating.Operation of the former method depends upon the variation of photonvelocity as a function of the energy per photon hence it is highlydependent upon appropriate materials being available. The latter methodutilizes interference between wavelets reflected from various portionsof a ruled surface. Because constructive interference can occur for anyintegral number of waves between adjacent grooves on the grating,spatial separation as a function of wavelength is not unique. Hence itis necessary to employ some form of order sorting" method used inconjunction with the grating.

Because the mechanical variability in the groove spacing and angle isusually associated with the fabrication of such grating, it'has not beenpossible to use such gratings under conditions in which the wavelengthbeing measured is in the visible region of the spectrum and 'where'thenumber of wavelength differences between adjacent grooves is large. An

exception to this is the use of a so-calIed echelle gratingfor farinfrared spectroscopy where the groove dimensions are su fficientlylarge that adequate tolerance can be obtained using machine tools intheir manufacture. Recently, however, new techniques for controlling theruling of gratings have made it possible to produce gratings withadequate precision to be used in high orders in the visible andultraviolet region of the spectrum. Spectrometers employing suchgratings have been investigated and constructed and instruments areavailable which use a combination of two spectrometers in series, one ofwhich is used to select the order of the other. The majority of theseinstruments however operate only under a small wavelength interval atany given setting. This is because conventional gratings capable ofyielding high resolution also produced an angular spread too large to becollected and focused conveniently.

A novel spectrometer employing an echelle grating has now been foundwhich is not subject to the deficiencies of the prior art.

SUMMARY OF THE INVENTION The novel spectrometer of the present inventionis comprised of the following essential components:

an entrance aperture at least a first collimating mirror a prism anechelle grating at least a first exit focal plane.

The grating is mounted in the spectrometer so that it rotates in twodirectionsi 1) around an axis parallel to the prism mounting axisparallel to the prism face, and 2) around an axis parallel to.thegrating rulings and thus perpendicular to the first grating mountingshaft, thus providing means for adjusting the vertical and horizontalcomponent of the dispersed energy in the focal plane.

By means of the aforementioned grating rotational abilities, thehorizontal and vertical components of the dispersed energy in the exitfocal plane can be adjusted independently,

thereby providing greater resolution and order spacing than hasheretofore been possible.

The prism'is rotatable around a horizontal axis which passes through thecentral ray of the incident beam and parallel to the first surface ofthe prism, i.e., the face on which the incident beam falls. By pivotingthe prism, the angle between the incident energy and the prism face canbe adjusted without appreciably altering the location of the incidentbeam on the prism face. By permitting only a single degree of rotationalfreedom in the prism insures that the dispersion of the prism is notinadvertantly added to the dispersion of the grating. Preferably, theface of the prism most distant from the grating is nondispersive, whichobviates the necessity of moving the collimating mirrors.

While the configuration of the entrance aperture is not critical, it ispreferred that the length is five to 10 times the width.

The superior resolution available with the novel spectrometer of thepresent invention can be illustrated with reference to the IronTripletfThe industry employs a rule of thumb indication of acceptability of aspectrometer by referring to the so-called Iron Triplet, i.e., a seriesof lines at 3,020 A. An instrument is considered satisfactory if threelines can be distinguished on a spectrograph with low powermagnification. By means of the high degree of resolution obtainable withthe novel device of the present invention, spectrographs are obtainablewhich show not only the three lines of the Iron Triplet" but actuallyfive lines, and which can be viewed without magnification.

By proper choice of prism geometry and material combined with an echellegrating having an appropriate number of grooves and groove spacing, aresolving power of 250,000 can be readily obtained throughout thespectral region from 1,200 A to 0.50 microns.

DESCRIPTION OF THE DRAWING Referring now to the drawing wherein:

FIG. 1 is an illustration in diagrammatical form of a plan view of thenovel spectrometer of the present invention showing the arrangement ofthe prism with respect to the grating.

FIG. 2 is an elevation view of the components of FIG. 1.

FIG. 3 is an illustration in diagrammatic form of the spectrometer ofthe present invention.

FIG. 4 is'an illustration in diagrammatic form of an optical system foruse with the spectrometer of the present invention for forming an imageof the grating surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,in FIGS. I & 2 there is shown in diagrammatic form the relationship ofthe prism to the echelle grating and the mounting and relative axes ofrotation of said prism and grating. Incident energy from a source (notshown) impinges upon prism 11 which is mounted on a prism mounting plate18 and which rotates on shaft 14. The incident energy passes throughprism 11 and is dispersed along a first coordinate extending in thedirection of the length of the slit. The thus-dispersed incident energyfrom the prism 11 is incident on echelle grating 12 which is mounted ongrating mounting plate 17 with shaft 16 parallel to the rulings on saidgrating 12 and wherein said grating mounting plate is connected to shaft15 which is perpendicular to shaft 16 and parallel to prism shaft 14.Shaft 14 is fastened to upright mounting plate 19 which is secured at aright angle to base 13. The defracted energy from the echelle grating l3now passes back through prism 11 where it is further refracted anddirected toward an output collimator (not shown). The arrangement of theprism and grating as shown in FIGS. 1 & 2 provides the maximumdispersion by both the prism and the echelle grating while at the sametime permitting a reasonable size for both the prism and grating andalso maintaining a compact beam which enables the instrument to be arelatively.

As shown in FIGS. 1 & 2 the prism and grating are supported by avertical mounting plate which in turn is supported by a base plate. Thebearings for the shaft are mounted upright in the vertical mountingplate. In an alternative embodiment, the base plate may be mounted inthe instrument frame by a shaft which rotates about a vertical axispassing through the intersection of the central ray of the incident beamand the first surface of the prism.

The collimating mirrors employed in the present invention are eitherspherical or an off-axis parabola.

in FIG. 3 there is illustrated in diagrammatic form the completespectrometer of the present invention. Energy from source 25 is focusedby optics 26 on aperture slot 21. The energy passing through aperture 21is incident on collimating mirror 22 which directs it on to a prism 11which disperses the energy which is then incident on echelle grating 12.Prism 11 rotates about shaft 14 while grating 12 rotates about shaft 16and shaft (not shown). The dispersed energy reflected from the groovefaces in grating 12 is returned through prism 11 to collimating mirror23 from whence it is focused at the exit focal plane 24.

In the above drawings the orientation of the grating is selected toreturn the energy directly from the grating to the output collimator bypassing it through the prism for a second time. In an alternativeconfiguration the orientation of the grating is such that the energyreflected from the grating is directed to the collimator without asecond pass through the prism. This alternative embodiment is desirablein applications where stray energy introduced by scatter from theincident beam of energy by the first surface of the prism isundesirable.

The novel spectrometer of the present invention, by reason of thearrangement of the prism and the grating with respect to each other,provides an essentially square focal plane for at least one spectraloctave. This yields, simultaneously, higher resolution and a broaderspectral range with desirable properties in that only a small range ofangles with respect to the central ray of the focal point is produced.

The novel spectrometer of the present invention has a focal planeconfiguration which is particularly suited to the use of two dimensionalelectrooptical sensor methods. As examples of suitable sensors which maybe employed in conjunction with the novel spectrometer of the presentinvention mention may be made of detector arrays, image tubes, imageintensifiers, image convertors, spatial/temporal encoders and opticalcorrelators.

Conventional methods for quantitating the output ofa spectrometer eitherutilize photographic film as an intermediate storage media withsubsequent quantitative densitometry of the film record or employ one ormore photoelectic sensors to produce an electrical output correspondingto the particular wavelength incident upon that sensor. Theconfiguration of the focal plane of conventional spectrometers does notpermit the use of two-dimensional electro-optical devices such as imagetubes conveniently. A major advantage of the type of display produced bythe novel echelle spectrometer of the present invention is that thetwo-dimensional nature of the display combined with the resolutionscaling introduced by successive orders permits the use oftwo-dimensional electro-optical quantitative sensors. An additionaladvantage of this type of spectrometer of the present invention is thata relatively small range of angles of the beams is involved in formingthe focal plane image; hence it is relatively simple to recollect theenergy following the initial spectral focal plane and perform additionalreimaging. This latter advantage is particularly useful in situationswhere it is desirable to modify the spectral data prior toelectro-optical detection. In cases where it is known that a widedynamic range of spectral components exists, appropriate filtering canbe introduced at the first spectral focal plane to compensate for thedynamic range and avoid local saturation of the detector and equalizethe intensity of the spectral lines of interest so that a sensor havinglimited dynamic range, such as an image tube, can be employed.

An alternative embodiment is to introduce into the first spectral focalplane a spatial/temporal encoder and then collecting the transmittedenergy on a single detector. This technique provides a unique timesequence code corresponding to each resolution element in the spectralfocal plane. The corresponding intensity may be decoded from the outputof the single detector by utilizing temporal autocorrelation. In theevent that only a small portion of the spectral data is desired,multiple reimaging can be utilized with a stationary transmission maskin the first spectral focal plane followed by reimaging into a secondspectral focal plane where the spatial/temporal encoder is located.

Referring now to FIG. 4 an example of the multiple reimaging is shown.The output energy from spectrometer 31 through spectral focal plane 30passes through condensing optics 32 to provide a first image 33 of thegrating face. The emitted beam then passes through reimaging optics 37to provide a second image 34 of the spectral focal plane and thence toreimaging optics 35 which provides a second image 36 of the gratingface. Optionally a spatial encoding disc or other type of encoder or animage tube may be employed at the spectral focal plane 36 or at asubsequent image of the spectral focal plane 34.

The multiple line, essentially square focal plane geometry provided bythe echelle spectrometer of the present invention also permits usage oftwo-dimensional electro-optical devices such as image intensifiers andimage converters in addition to conventional image transducers such asvidicon or image orthicon. These devices may be used to enhance thesensitivity of the system or to extend the spectral range of the imagetube.

Two-dimensional incoherent optical filtering methods, i.e., an opticalcorrelator, may also be used with the type ofdisplay generated by thespectrometer of the present invention. Optical correlation utilizingmany spectral lines per element provides enhanced sensitivity overconventional techniques which employ only one spectral line per element.

The basic capability of the spectrometer to observe and detect a broadrange of the spectrum at high resolution is useful whether the energybeing analyzed is due to emission by a material or due to absorption bythe material from a known source. Particular instrument configurationsmay be suited to specific problems but the basic advantages of thisinstrument approach are the two-dimensional nature of the focal planeand the resolution scaling introduced as various orders are observed.

A suitable echelle grating for use in the present invention is comprisedof 73.25 lines per millimeter, with a groove face blaze angle ofapproximately 63. A 3060-90 prism of cal- .cium fluoride provides thenecessary order separation and broad spectral coverage. Focal lengthsand slit sizes are selected based on the particular instrumentconfiguration and the purpose for which the instrument is to beemployed.

This dispersing system may be combined for example, with 1 meter focallength optics to yield an instrument having reciprocal dispersion of 1.4angstroms per mm at a wave length ofapproximately 5,000 A, or 0.7 a/mm.at 2,500 A, and covering the spectral range from 1,500 A to 6,000 A on asingle 4 X 5 plate in the focal plane. Resolution is approximately 0.03A at 2,500 A with a 50 micron slit width.

For use with an image tube such as a vidicon, the focal length of theoutput collimator is preferably reduced to approximately l25 mm. Thisyields a reciprocal dispersion of l 1.2 A/mm or a resolution ofapproximately 0.5 A with a conventional vidicon.

A very compact system compatible with packaging for an electronics rackis obtained with 0.5 meter focal length optics. The reciprocaldispersion is comparable to that obtained with a conventional 3 meterinstrument (2.8 A per mm) yet it occupies less thana foot of verticalspace, and less than 2 feet of depth. Resolution is approximately 0.]angstroms at 5,000 A (limited by optical aberrations).

Unlike prior art devices, the spectrometer of the present invention iscapable of simultaneous coverage of a wavelength range exceeding afactor of 100 to l with an essentially constant ratio between spectralresolution and wavelength. It is also unique in that the range ofdispersion angles remain small; hence it permits a compact equipmentconfiguration. For example, in the near ultra-violet, results comparableto a meter prior art device are obtainable with a 1 meter device of thepresent invention.

I claim:

1. A spectrometer comprising:

a. an entrance aperture;

b. first collimating means;

c. a dispersing prism having at least one face;

d. an echelle grating;

e. a first shaft, said first shaft being parallel to said prism face;

f. a second shaft, said second shaft being parallel to the gratingrulings and perpendicular to the first shaft;

g. means for mounting said grating for rotation about said first andsecond shafts; and

h. means to form at least a first exit focal plane whereby radiationtransmitted by the collimating means impinges on said prism, saidradiation is dispersed, the dispersed radiation from the prism strikesthe grating wherein horizontal and vertical components of said radiationare ultimately focused at the exit focal plane.

2. The spectrometer of claim 1 wherein means are provided to rotate saidprism around an axis passing through the central ray of the incidentbeam and parallel to the said prism face.

3. The spectrometer of claim 1 which includes means to form amultiplicity of exit focal planes, each of said focal planes beingessentially square and capable of covering at least one spectral octave.

4. The spectrometer of claim 3 which includes a spectral filter in atleast one of said focal planes.

5. The spectrometer of claim 3 which includes a spatial filter in atleast one of said focal planes.

6. The spectrometer of claim 3 which includes a detector for the emittedradiation in at least one of said focal planes.

7. The spectrometer of claim 6 wherein said detector is atwo-dimensional electrooptical detector.

8. The spectrometer of claim 7 wherein said detector is a vidicon tube.

9. The spectrometer of claim 1 which includes a focusing mirror andwherein said grating is positioned such that the incident radiation onsaid grating is directed to said focusing mirror by passing through saidprism.

10. The spectrometer of claim 1 which includes a detector for theemitted radiation in said focal plane.

11. The spectrometer of claim 1 wherein the focal length optics of saiddevice are 0.5 meter and the resolution is about 0. l angstroms at 5,000angstroms.

12. The spectrometer of claim 1 wherein said focal plane is followed bymeans to form an image of the grating surface.

13. The spectrometer of claim 12 wherein a detector is located at saidimage.

14. The spectrometer of claim 1 wherein said prism is a 30, 60", and 90prism.

15. The spectrometer of claim 1 which includes means to rotate theprism.

16. The spectrometer of claim 1 wherein the prism face is nondispersiveand further said face is positioned to receive the radiation impingingon the prism.

17. The spectrometer of claim 1 in which said means to form at least afirst exit focal plane includes second collimating means whereby whenthe grating is rotated about its first and second shafts, the horizontaland vertical components from the grating pass through the prism, strikethe second collimating means, and are focused at the exit focal plane.

18. The spectrometer of claim 1 wherein the first and second shafts onwhich the grating is mounted are rotatable.

19. The spectrometer of claim 1 wherein the collimating means includes acollimatingmirror. 20. A spectrometer comprising:

a. an entrance aperture;

b. a first collimating mirror;

c. a dispersing prism having at least one nondispersive face;

d. means to rotate the prism about an axis which is parallel to saidface;

e. an echelle grating;

f. a rotatable first shaft, said first shaft being parallel to saidprism face;

g. a rotatable second shaft, said second shaft being parallel to thegrating rulings and perpendicular to the first shaft;

h. means for mounting said grating for rotation about said first andsecond shafts; and

i. a second collimating mirror to form at least a first exit focal planewhereby radiation reflected by the first collimating mirror impinges onsaid nondispersive prism face, the dispersed radiation passing throughthe prism strikes the grating wherein horizontal and vertical componentsof said radiation pass first through the prism, subsequently strike thesecond collimating mirror, and are focused at the exit focal plane.

21. A spectrometer comprising:

a. an entrance aperture;

b. means to redirect radiation;

c. a dispersing prism having at least one face;

d. an echelle grating;

e. a first shaft, said first shaft being parallel to said prism face;

f. a second shaft, said second shaft being parallel to the gratingrulings and perpendicular to the first shaft; g. means for mounting saidgrating for rotation about said first and second shafts; and 1 h. atleast a first exit focal plane whereby radiation is directed to impingeon said prism, said radiation is dispersed, the dispersed radiation fromthe prism strikes the grating wherein horizontal and vertical componentsof said radiation are ultimately received at the exit focal plane.

22. The spectrometer of claim 21 wherein means are provided to rotatesaid prism around an axis passing through the central ray of theincident beam and parallel to the said prism face.

23. The spectrometer of claim 21 which includes means to form amultiplicity of exit focal planes, each of said focal planes beingessentially square and capable of covering at least one spectral octave.

1. A spectrometer comprising: a. an entrance aperture; b. first collimating means; c. a dispersing prism having at least one face; d. an echelle grating; e. a first shaft, said first shaft being parallel to said prism face; f. a second shaft, said second shaft being parallel to the grating rulings and perpendicular to the first shaft; g. means for mounting said grating for rotation about said first and second shafts; and h. means to form at least a first exit focal plane whereby radiation transmitted by the collimating means impinges on said prism, said radiation is dispersed, the dispersed radiation from the prism strikes the grating wherein horizontal and vertical components of said radiation are ultimately focused at the exit focal plane.
 2. The spectrometer of claim 1 wherein means are provided to rotate said prism around an axis passing through the central ray of the incident beam and parallel to the said prism face.
 3. The spectrometer of claim 1 which includes means to form a multiplicity of exit focal planes, each of said focal planes being essentially square and capable of covering at least one spectral octave.
 4. The spectrometer of claim 3 which includes a spectral filter in at least one of said focal planes.
 5. The spectrometer of claim 3 which includes a spatial filter in at least one of said focal planes.
 6. The spectrometer of claim 3 which includes a detector for the emitted radiation in at least one of said focal planes.
 7. The spectrometer of claim 6 wherein said detector is a two-dimensional electrooptical detector.
 8. The spectrometer of claim 7 wherein said detector is a vidicon tube.
 9. The spectrometer of claim 1 which includes a focusing mirror and wherein said grating is positioned such that the incident radiation on said grating is directed to said focusing mirror by passing through said prism.
 10. The spectrometer of claim 1 which includes a detector for the emitted radiation in said focal plane.
 11. The spectrometer of claim 1 wherein the focal length optics of said device are 0.5 meter and the resolution is about 0.1 angstroms at 5,000 angstroms.
 12. The spectrometer of claim 1 wherein said focal plane is followed by means to form an image of the grating surface.
 13. The spectrometer of claim 12 wherein a detector is located at said image.
 14. The spectrometer of claim 1 wherein said prism is a 30*, 60*, and 90* prism.
 15. The spectrometer of claim 1 which includes means to rotate the prism.
 16. The spectrometer of claim 1 wherein the prism face is nondispersive and further said face is positioned to receive the radiation impinging on the prism.
 17. The spectrometer of claim 1 in which said means to form at least a first exit focal plane includes second collimating means whereby when the grating is rotated about its first and second shafts, the horizontal and vertical components from the grating pass through the prism, strike the second collimating means, and are focused at the exit focal plane.
 18. The spectrometer of claim 1 wherein the first and second shafts on which the grating is mounted are rotatable.
 19. The spectrometer of claim 1 wherein the collimating means includes a collimating mirror.
 20. A spectrometer comprising: a. an entrance aperture; b. a first collimating mirror; c. a dispersing prism having at least one nondispersive face; d. means to rotate the prism about an axis which is parallel to said face; e. an echelle grating; f. a rotatable first shaft, said first shaft being parallel to said prism face; g. a rotatable second shaft, said second shaft being parallel to the grating rulings and perpendicular to the first shaft; h. means for mounting said grating for rotation about said first and second shafts; and i. a second collimating mirror to form at least a first exit focal plane whereby radiation reflected by the first collimating mirror impinges on said nondispersive prism face, the dispersed radiation passing through the prism strikes the grating wherein horizontal and vertical components of said radiation pass first through the prism, subsequently strike the second collimating mirror, and are focused at the exit focal plane.
 21. A spectrometer comprising: a. an entrance aperture; b. means to redirect radiation; c. a dispersing prism having at least one face; d. an echelle grating; e. a first shaft, said first shaft being parallel to said prism face; f. a second shaft, said second shaft being parallel to the grating rulings and perpendicular to the first shaft; g. means for mounting said grating for rotation about said first and second shafts; and h. at least a first exit focal plane whereby radiation is directed to impinge on said prism, said radiation is dispersed, the dispersed radiation from the prism strikes the grating wherein horizontal and vertical components of said radiation are ultimately received at the exit focal plane.
 22. The spectrometer of claim 21 wherein means are provided to rotate said prism around an axis passing through the central ray of the incident beam and parallel to the said prism face.
 23. The spectrometer of claim 21 which includes means to form a multiplicity of exit focal planes, each of said focal planes being essentially square and capable of covering at least one spectral octave. 